1. Field of the Invention
This invention relates to a container for a photographic film cartridge which is characterized by the composition of its cap.
2. Description of the Prior Art
A conventional cap of the container for the photographic film cartridge was usually made of high-pressure branched low-density polyethylene (LDPE) resin having 0.927 g/cm.sup.3 in density (ASTM D 1505) and 4.0 g/10 minutes in melt index (ASTM D 1238, at 190.degree. C., MI). This resin is superior in flexibility at a low temperature, fitness of cap and body, compressive strength, a few burrs and gate marks.
However, in the case of using the LDPE resin, molding troubles, such as a short shot of the molten resin, warp, twist and deformation, frequently occur. Its insufficient heat stability was also a problem. That is, the LDPE resin staying in a continuous molding machine at its screw, manifold, hot runner or other places was gradually colored brown or dark brown by heat. This colored resin was gradually extruded to cause coloring troubles. The generation rate of the colored products was high, such as 3 to 10%, and these products should be extracted by a checker or a checking machine. A more important problem was that when coloring troubles happened, the molding machine must be disassembled and washed to remove the colored resin completely. Much effort and a long time were spent for this cleaning work. As another problem, since the fluidity of the LDPE resin was not sufficient because of its low MI, its temperature should be high at molding. As a result, molding cycle became lengthy.
In order to solve the coloring problem, when the cap was colored by blending carbon black, not only did the appearance of the container became unfavorable but also the manufacturing cost became elevated. Furthermore, when the whole container was colored black, the inside of the container became hot in the sunshine. This caused degradation of the photographic film in the container.
A metal container is also known (e.g. Japanese Utility Model KOKOKU No. 58-46413). However, the metal container was expensive, and its mass-producibility was inferior compared to a plastic container. Accordingly, it is not now utilized.